Wireless communication receivers are used in applications such as wireless sensor networks, point-to-point data links, mobile phones, wireless local area networks, mobile WiMAX, mobile wireless infrastructure, and data-streaming applications. Receivers designed for such applications desirably feature very low power consumption (e.g., less than 15 mW), very small die area, very low cost (e.g., less than $1), and increasingly higher levels of integration of their digital signal processing functions. These characteristics permit the deployment of very-large-scale battery-operated networks of devices implementing such applications. A direct-conversion receiver is one possible implementation of a wireless communication receiver that may possess at least some of these characteristics. A direct-conversion receiver can lower power consumption by down-converting an incoming wireless signal directly to the base-band, thereby allowing the base-band filtering chain to operate at frequencies lower than otherwise possible. A direct-conversion receiver also offers superior receiver-blocking resilience due to the absence of any image-rejection constraints, which are common in other receiver architectures, such as low-intermediate-frequency receivers.
Unfortunately, direct-conversion receivers may also suffer from certain drawbacks. For example, a DC offset introduced into the direct-conversion receiver may cause the receiver to perform sub-optimally by, for example, preventing a signal from being received or degrading the receiver's dynamic range. There are two kinds of DC offsets: static and dynamic. A static DC offset may be caused by circuit mismatches in the receiver's circuits, or by the local oscillator self-mixing at the inputs of the receiver's mixer. A dynamic, time-varying offset may be caused by the local-oscillator signal radiating proximate to the receiver. The dynamic time-varying offset can change unpredictably over time with an unidentifiable amplitude and frequency. The local-oscillator signal may be radiated by the receiver itself and reflect back to it from nearby surfaces, thereby self-mixing in the direct-conversion receiver mixer stage. In addition, a similar modulated local oscillator signal may be caused by a nearby transmitter operating on the same channel as the receiver, particularly if both the receiver and transmitter operate in an unlicensed, uncontrolled radio frequency (RF) band, such as the Industrial, Scientific, and Medical Band. This transmitted modulated local oscillator signal may mix with the local oscillator of the receiver in the receiver mixer stage. Another source of a dynamic, time-varying offset is coupling of the local-oscillator signal to the inputs of the low-noise amplifier at the receiver input. Due to the high gain of the low-noise amplifier, the coupled local-oscillator signal may be amplified to a level sufficient to self-mix in the direct-conversion mixer stage.
Dynamic DC offsets should be cancelled while the desired signal is being received, and previous implementations of direct-conversion receivers have utilized limited offset-cancellation circuitry. These implementations, however, are generally unsuitable for low-cost, low-power wireless applications. Some implementations, for example, employ analog DC offset-correction circuitry, which may be too large, consume too much power, and/or require too much time to cancel the DC offsets. Other implementations correct only for static DC offsets, or do not continually re-calibrate their corrections, thereby ignoring dynamic time-varying offsets. Finally, some implementations do not account for changes in dynamic DC offsets in the presence of gain changes in a receiver's front-end low-noise amplifier. Clearly, a need exists for receivers that offer extremely low power consumption, very small silicon area, and fast and efficient cancellation of static and dynamic DC offsets.